Thermal treatment of water-soluble particles formed by compounds composed of carbon nanobelts and C60 molecules

It was previously shown that spherical particles are self-assembled by compounds composed of C60-(6,6)CNB-C60, where CNB stands for “carbon nanobelt”, by mixing two individual solutions of C60 and (6,6)CNB molecules dissolved in 1,2-dichlorobenzene at room temperature. The particles are monodisperse in water thanks to their high absolute value of the zeta potential in water. In this report, we investigate the effect of thermal treatment of the particles on some changes in the physical properties and structures. We find that the particles become electrically conductive after thermal treatment at 600 °C for 1 h. We suppose that the change in the electrical characteristics might have been caused by the structural change of (6,6)CNBs into opened-up ribbons composed of fused benzene rings, which construct networks supported by C60 molecules in the particles, judging by the change in the absorption and mass spectra of the particles after thermal treatment and analysis of a possible change in the structure of C60-(6,6)CNB-C60 based on quantum chemical calculations employing the PM6 method, with which it is known that nanostructures such as carbon nanotubes (CNTs) and (6,6)CNBs can be correctly estimated.

the particles before and after thermal treatment are shown in Fig. 2, where Fig. 2a,b, respectively, represent the particles without thermal treatment and those after thermal treatment at 600 °C.A charge-up (charge accumulation) phenomenon was observed on the surface of the particles before thermal treatment (see Fig. 2a), whereas there was no charge accumulation on the surface of the particles after thermal treatment at 600 °C (Fig. 2b).This suggests that the electrical characteristics of the particles might have been changed by thermal treatment of the particles at 600 °C, which will be discussed later.
The diameter of a particle, and the hydrodynamic diameter and zeta potential of a particle dispersed in distilled water before and after thermal treatment at 600 °C are shown in Table 1.The diameter of a particle slightly decreased after thermal treatment.The decrease in the diameter of a particle coincides with the decrease in the weight of a particle during thermal treatment; that is, w 2 /w 1 ≈ d 2 /d 1 3 , where w and d are the weight and diameter of a particle and subscripts 1 and 2 represent before and after thermal treatment (see Fig. S1 in the Supplementary Information for the time variation of the temperature and the weight of the particles during thermal treatment).Note that there was no significant weight loss during thermal treatment of the particles at 300 °C for 1 h 16 and therefore it is supposed that the present weight loss was not caused by the evaporation of the solvent captured in the particles.Even after thermal treatment, the absolute value of the zeta potential of a particle dispersed in distilled water was as high as 35.4 mV and as a result, the particles were monodisperse in water.The particles finally precipitated in distilled water due to gravity, but once the suspension having been shaken, the particles monodispersed again thanks to the high absolute value of the zeta potential in water (see   Fig. S3 and Video S1 in the Supplementary Information, respectively, for the time variation of the turbidity of the suspension and for the redispersion of the particles after a shake of the suspension).The current-voltage (I-V) characteristics of the particles, which were measured using an atomic force microscope (AFM) 22,23 , are shown in Fig. 3, where the current is averaged over five consecutive measurements, noting that there was no clear hysteresis in the I-V curves, and AFM images of the particles are also shown, the red spots on the surface of the particles representing the points contacted by a conductive diamond probe.As is clearly shown, there was no current through the particle before thermal treatment, whereas the particle became electrically conductive after thermal treatment at 600 °C.Let us make a rough estimate of the conductivity of the particles after thermal treatment.The radius of the tip of the conductive diamond probe, r , the electric potential difference, �φ , the electric current, I , and the diameter of a particle, d , being 10 nm, 5 V, 15 nA and 440 nm, the conductivity of the particle, σ = i/E , where i = I/ πr 2 and E = �φ/d , are the current density and electric field, would be of the order of 5 S m −1 .Note that the charge-up phenomenon was still observed with SEM microscopy in the case of the particles after thermal treatment at 500 °C for 1 h (Fig. S3 in the Supplementary Information for an SEM image of particles after thermal treatment at 500 °C) and it is therefore supposed that the insulatorconductor transition occurred between 500 and 600 °C.
As explained above, the particles were originally formed by compounds composed of one (6,6)CNB and two C 60 molecules; that is, C 60 -(6,6)CNB-C 60 , (see Fig. 1), and therefore, it is supposed that the alteration in the electrical characteristics after thermal treatment at 600 °C might have been caused by some structural change of the compounds in the particles.However, there were no dramatic changes in the TEM images, and SAED and XRD patterns (see Figs. S4 and S5 in the Supplementary Information for the TEM images/SAED patterns and XRD patterns before and after thermal treatment at 600 °C).The absorption spectra of particles before and after thermal treatment are shown in Fig. S6 in the Supplementary Information.The wavelength corresponding to the absorption peak was 204 nm in the case of the particles without thermal treatment, whereas the peak disappeared after thermal treatment at 600 °C, noting that the particles before thermal treatment were composed of compounds; C 60 -(6,6)CNB-C 60 , in which C 60 and (6,6)CNB are bonded via charge transfer, and the absorption wavelength corresponds to charge transition 16 .It is, therefore, supposed that the structure of the compounds; C 60 -(6,6)CNB-C 60 , might have been changed after thermal treatment.According to the TOF-MS of the particles, (6,6)CNBs and C 60 molecules were detected before thermal treatment, whereas there was no (6,6)CNBs observed, although C 60 was detected, after thermal treatment at 600 °C (see Fig. S7 in the Supplementary Information for the TOF-MS), which suggests that (6,6)CNB might have been broken or dissociated during thermal treatment at 600 °C for 1 h.We carried out quantum chemical calculations concerning a structural change from a (6,6)CNB to a carbon nano ribbon (CNR) [24][25][26][27][28] disconnecting a pair of fused benzene rings by semi-empirical PM6 29 .It is known that the structures of carbon nanostructures such as carbon nanotubes (CNTs) and CNBs can be correctly estimated with the PM6 method 16,[30][31][32] .In the present study, the most stable structure obtained with PM6 among opened-up ribbons was the one terminated with a square, which coincided with the structure obtained with the density functional theory (DFT) (see Fig. S8).The details of the structures of the opened-up ribbon obtained with PM6 and DFT are also summarised in Fig. S8 in Fig. 4. The structures formed by compounds; C 60 -(6,6)CNB-C 60 , which corresponds to those before thermal treatment, are also shown in Fig. S9.The configuration formed by one ribbon and two C 60 molecules is shown in Fig. 4a.Some part of the ribbon is positioned between two C 60 molecules.As the number of C 60 and CNBs molecules increases, the configurations become more complicated (Fig. 4b,c).Those compounds were not regularly located in the particles both before and after thermal treatment (see Figs. 4 and S9), which explains why there were no dramatic changes in the TEM images, and SAED and XRD patterns (Figs.S4 and S5).It is supposed that the in-ribbon electric conductivity is high in the longitudinal direction 28,[33][34][35] , whereas the conductivity www.nature.com/scientificreports/across two layers of ribbons is low [33][34][35] .The overall conductivity is determined by both in-ribbon and crossribbons electron transports and in the present case, the conductivity was of the order of 5 S m -1 .We suppose that (6,6)CNBs may be changed to opened-up ribbons as mentioned above and particles are filled with ribbon/ C 60 complexes, the ribbons constructing networks supported by C 60 molecules during thermal treatment, which explains the disappearance of charge accumulation on the surface of the particles (see Fig. 2b), the appearance of conductivity of the particles [33][34][35][36][37][38][39][40][41] (Fig. 3), the disappearance of the peak in the absorption spectrum (Fig. S6) and the disappearance of (6,6)CNBs in the mass spectrum (Fig. S7) after thermal treatment of particles at 600 °C.Networks of ribbons can be constructed in particles via pyrolysis as shown in the present study, but we suppose that networks may also be created on the surface of particles via photolysis with irradiation of ultraviolet (UV) laser beams.It would also be possible to attract compounds; C 60 -(6,6)CNB-C 60 , to an anode in a dc electric field, noting that C 60 is negatively charged in the compound 16 , and then the (6,6)CNBs would be opened up via either pyrolytic or photolytic treatment of the compounds (see Fig. 4a), which may lead to the development of nano circuits technology.The particles are monodisperse in water even after thermal treatment, which means that the particles can be used as conductive colloidal ones.

Conclusion
We produced water-soluble particles of a uniform diameter composed of compounds; C 60 -(6,6)CNB-C 60 , at room temperature by mixing two individual solutions of carbon nanobelts and C 60 molecules dissolved in 1,2-dichlorobenzene.We then found that the particles become electrically conductive after thermal treatment of the particles at 600 °C for 1 h.The mechanism of the change in the electrical characteristics after thermal treatment is an open question, but we suppose that the change in the electrical conductivity of particles after thermal treatment might have been caused by the structural change of carbon nanobelts into opened-up ribbons of fused benzene rings, which construct networks assisted by C 60 molecules.The creation of nano/micro structures and materials using basic carbon nano units such as CNB, CPP and fullerenes such as C 60 , C 70 and C 59 N 42,43 is our future task.

Methods
Synthetic procedure of particles 16 (a) (6,6)CNBs (Tokyo Chemical Industry Co. Ltd.) and C 60 molecules (Kanto Chemical Co.Inc.) were individually dissolved in 1,2-dichlorobenzene.The molar concentrations of (6,6)CNBs and C 60 molecules dissolved in 1,2-dichlorobenzene were, respectively, set at 0.7 and 1.4 µmol ml -1 .(b) Those two solutions were mixed at room temperature, noting that the molar concentrations of (6,6)CNBs and C 60 molecules dissolved in 1,2-dichlorobenzene became 0.35 and 0.7 µmol ml -1 after the mixture of the solutions.(c) The mixed solution was left still for 1 week in a dark box at room temperature.(d) 1,2-dichlorobenzene was then replaced by ethanol, followed by sonication and centrifugation twice.(e) The particles separated by centrifugation were dispersed in distilled water, followed by sonication.

Thermal treatment of particles
(a) The above particles were placed in a thermogravimetric (TG) analyser (DTG-60, Shimadzu Corp.).(b) The temperature was raised up to 600 °C at a rate of 15.9 K min -1 and then kept at 600 °C for 1 h with the flow of N 2 gas.Then, the temperature was decreased naturally down to room temperature.The time variation of the temperature and the weight of the particles was recorded.The weight was calibrated with a precision scale (Excellence plus XP56, Mettler-Toledo International Inc.).

Observation and characterisation of the particles
(a) The structures of the particles were observed by SEM (SU8030, Hitachi Ltd.) and TEM (2200FS, JEOL Ltd. and ARM200F, JEOL Ltd.), where SAED patterns were also obtained.(b) The size of the particles was measured, targeting at 100 particles from the SEM images.(c) The hydrodynamic diameter and zeta potential of the particles dispersed in distilled water were measured by Zetasizer (Nano-ZS, Malvern Panalytical Ltd.).(d) The precipitation process of the particles dispersed in distilled water was observed and recorded on the hard disc of a computer.The intensity of the transmitted light of 700 nm wavelength through the whole solution confined in a glass container was measured with a spectral photometer (U-3500 Spectrophotometer, Hitachi High-Tech Co.) and the turbidity, which was defined by (1 − I trans /I in ) × 100% , where I in and I trans are, respectively, the intensities of the incident and transmitted light, was obtained.(e) The current-voltage (I-V) characteristics of the particles were measured using an atomic force microscope (AFM) (Cypher, Oxford Instruments PLC.).First, a droplet of the suspension of the particles dispersed in ethanol was dropped onto the surface of a substrate, which was composed of platinum and titanium films deposited on a mica base (see Fig. 5) and then ethanol was evaporated naturally.The substrate, on the surface of which the particles were placed, was set on a metal disc.The metal disc and the platinum film were connected by an aluminium foil and silver paste.The surface morphology of the particles was measured

Figure 2 .
Figure 2. SEM images of particles formed by compounds composed of (6,6)CNBs and C 60 molecules.The particles were synthesised by mixing two solutions of (6,6)CNBs and C 60 molecules dissolved in 1,2-dichlorobenzene.The molar concentrations of (6,6)CNBs and C 60 molecules were 0.35 and 0.70 µmol ml −1 after the mixture of two solutions.The particles were thermally treated at 600 °C for 1 h.The scale bars represent 5 µm.(a) Particles without thermal treatment.A charge-up phenomenon occurred.(b) Particles after thermal treatment at 600 °C.There was no charge-up.

Figure 3 .
Fig.S3and Video S1 in the Supplementary Information, respectively, for the time variation of the turbidity of the suspension and for the redispersion of the particles after a shake of the suspension).The current-voltage (I-V) characteristics of the particles, which were measured using an atomic force microscope (AFM)22,23 , are shown in Fig.3, where the current is averaged over five consecutive measurements, noting that there was no clear hysteresis in the I-V curves, and AFM images of the particles are also shown, the red spots on the surface of the particles representing the points contacted by a conductive diamond probe.As is clearly shown, there was no current through the particle before thermal treatment, whereas the particle became electrically conductive after thermal treatment at 600 °C.Let us make a rough estimate of the conductivity of the particles after thermal treatment.The radius of the tip of the conductive diamond probe, r , the electric potential difference, �φ , the electric current, I , and the diameter of a particle, d , being 10 nm, 5 V, 15 nA and 440 nm, the conductivity of the particle, σ = i/E , where i = I/ πr 2 and E = �φ/d , are the current density and electric field, would be of the order of 5 S m −1 .Note that the charge-up phenomenon was still observed with SEM microscopy in the case of the particles after thermal treatment at 500 °C for 1 h (Fig.S3in the Supplementary Information for an SEM image of particles after thermal treatment at 500 °C) and it is therefore supposed that the insulatorconductor transition occurred between 500 and 600 °C.As explained above, the particles were originally formed by compounds composed of one (6,6)CNB and two C 60 molecules; that is, C 60 -(6,6)CNB-C 60 , (see Fig.1), and therefore, it is supposed that the alteration in the electrical characteristics after thermal treatment at 600 °C might have been caused by some structural change of the compounds in the particles.However, there were no dramatic changes in the TEM images, and SAED and XRD patterns (see Figs. S4 and S5 in the Supplementary Information for the TEM images/SAED patterns and XRD patterns before and after thermal treatment at 600 °C).The absorption spectra of particles before and after thermal treatment are shown in Fig.S6in the Supplementary Information.The wavelength corresponding to the absorption peak was 204 nm in the case of the particles without thermal treatment, whereas the peak disappeared after thermal treatment at 600 °C, noting that the particles before thermal treatment were composed of compounds; C 60 -(6,6)CNB-C 60 , in which C 60 and (6,6)CNB are bonded via charge transfer, and the absorption wavelength corresponds to charge transition16 .It is, therefore, supposed that the structure of the compounds; C 60 -(6,6)CNB-C 60 , might have been changed after thermal treatment.According to the TOF-MS of the particles, (6,6)CNBs and C 60 molecules were detected before thermal treatment, whereas there was no (6,6)CNBs observed, although C 60 was detected, after thermal treatment at 600 °C (see Fig.S7in the Supplementary Information for the TOF-MS), which suggests that (6,6)CNB might have been broken or dissociated during thermal treatment at 600 °C for 1 h.We carried out quantum chemical calculations concerning a structural change from a (6,6)CNB to a carbon nano ribbon (CNR)[24][25][26][27][28] disconnecting a pair of fused benzene rings by semi-empirical PM629 .It is known that the structures of carbon nanostructures such as carbon nanotubes (CNTs) and CNBs can be correctly estimated with the PM6 method16,[30][31][32] .In the present study, the most stable structure obtained with PM6 among opened-up ribbons was the one terminated with a square, which coincided with the structure obtained with the density functional theory (DFT) (see Fig.S8).The details of the structures of the opened-up ribbon obtained with PM6 and DFT are also summarised in Fig.S8.The structures formed by ribbon/C 60 complexes are shown

EFigure 5 .
Figure 5. Outline of the measurement system of the I-V characteristics of particles.Particles were placed on a platinum film and the current is measured using a diamond conductive probe imposing a force of 10 nN onto the top of a particle.

Table 1 .
Diameter of a particle, and the hydrodynamic diameter and zeta potential of a particle dispersed in distilled water before and after thermal treatment.